

The new header is added to the list of headers. It is automatically saved to your preferences so that every time you open the modifyheaders dialog it will be available.

Notice that a green circle is displayed on the right. This indicates that the header is enabled and will be sent with every request while the modifyheaders dialog is open. To disable the header, highlight the line of that header and click the Enable/Disable button.
Alternatively for convenience, double click on a line and it will toggle the status. A red circle indicates the header is disabled.